Fungus gnats

Fungus gnats are members of the Sciaridae family. They’re small (3-5 mm), dark, mosquito-like flies with long, slender antennae and long legs. It is particularly the Fungus gnat larvae that do the most damage.

Biology

Fungus gnats are especially fond of warm, moist environments in the proximity of plants. They can also survive the whole year round in greenhouses.

After mating the females lay 50-200 eggs that hatch within 2-3 days. The larvae over the course of a 2-3 week period go through four stages, at the end of which they are around 5 mm long, transparent white and have a noticeable black head.

The larvae pupate in the soil and appear three days later as adult gnats. With temperatures higher than 24° C, they reproduce continuously and with a 3-4 week life cycle, rapidly. The larvae feed not only on dead organic material such as moulds and algae, but also on living material like root- and stem tissue. They bore into the root and/or stem of cuttings, seedlings and young plants. Thanks to the feeding damage, a number of routes to secondary infection are opened up, giving plant diseases such as Pithium, Phytophtora, Botrytis, Fusarium and Verticillium an opportunity.

Damage

The larvae can cause direct damage by nibbling on roots and stems – they can even bore right the way through them. Indirect damage is also caused thanks to the larvae spreading nematodes, mites, mould spores and viruses.

Combating

Clear away dead organic material and use good quality compost.

Hypoaspis miles: The soil dwelling predatory mite Hypoaspis miles is a specialist in hoovering up Fungus gnat larvae.

Atheta coriaria: The predatory beetle Atheta coriaria is an extremely voracious and efficient predator of Fungus.

Steinernema-System: The Steinernema-System (using nematodes) can be used successfully against Fungus gnat

Nematoda: Round worms or nematodes invade the eggs and larvae of Fungus gnats to reproduce themselves.